You might want to compare them to a flying mammal, a bat. They tend to have a wingspan of about 1-6, drawing on my vague knowldge of that stupid measurement system... for every 1ft of body length theres about 6ft of wingspan. A fruitbat that lives in Indonesia has a wingspan of 6ft across and some of the big buggers that fly over my house in spring and autumn get up to about 3ft across.
Figure your human sized half celestial would be anywhere between 5-7ft tall, so that would give a wingspan of anywhere between 30-40 or so feet across. Which I figure is about right.
As for humans not being able to fly, same goes for Mr Dragon, but its a fantasy world where people huck fireballs at each other and shrug off damage that would wreck a car, its not really the place to have a lot of physics.
Of course, you can fold wings. Probably at the first 'elbow' joint which is about 2/5th's of the way up on each wing and a smaller joint equal to a 'wrist' which is about 4/5th's of the way up near the tip of the skeletal structure. So why your celestial may not be the tallest of critters he/she is likely to have about their height again on top and quite a considerable upper bodystrength, not to mention that that mass of feathers is probably quite bulky.
Oh, as a half-celestial you cant hover, you havent the manuverability class to do so (Fly- Good: Not sure if it lets you hover), a Monster feats to improve your airbourne manueverability by 1 scale up should let you be able to hover.
Anyway, enough musings on celestials before I go to bed.
Wonder if they lay eggs like a monotreme mammal?
